The present invention relates to temperature sensitive electrical switching devices and, more particularly, to an improved switching device construction which provides for simple, accurate calibration.
It is desirable to protect electrical devices, such as motors, generators, and transformers, from the effects of overheating. While power supply lines circuit breakers provide protection from excessive currents for such electrical devices, circuit breakers do not protect against overheating which may occur during continuous operation of a device at a current level which is not excessive. To provide adequate thermal protection for an electrical device, a thermally responsive switch may be placed within the device to monitor the temperature of the device. Such a switching device may be of the type which completes an electrical circuit when the temperature of the device exceeds a predetermined threshold temperature; alternatively, it may be of the type in which an electrical circuit is broken when the temperature of the device exceeds the predetermined threshold temperature.
In order for a thermostatic switch to be positionable within many electrical devices, however, it is necessary that the thermostatic switch be relatively small in size. In fabricating such a miniaturized thermostatic switch, accurate positioning of the switch elements to afford precise calibration of the switching device has been difficult to achieve.
Many prior art switches include a thermostatic blade having a first electrical contact and a second, stationary electrical contact. Some switching devices have been calibrated by manually adjusting the position of the second electrical contact, either by means of an adjusting screw or, alternatively, by denting the switch casing so as to move the second electrical contact into its desired position. Neither of these techniques has been particularly advantageous.
Calibration of switches incorporating an adjusting screw for positioning the second electrical contact may be performed in an oven with the switch heated to the desired threshold temperature level. It will be appreciated that the required manual calibration may result in a substantial percentage of the switches being inaccurately calibrated, depending on the skill of the technician performing the calibration operation. Additionally, this calibration technique is relatively expensive due to the labor costs involved. On the other hand, while denting the switch casing to move the second contact into its final desired position may be accomplished more rapidly by a technician, the final calibration achieved is not always as precise as may be desired.
One approach to the solution of the calibration problem of thermal responsive switching devices is described in U.S. Pat. No. 3,230,607, issued Jan. 25, 1966, to Gelzer. The Gelzer patent discloses a switch in which a bimetal blade is connected to one of a pair of electrically conductive posts extending through a nonconductive mounting structure. In operation, bimetal blade cooperates with a stationary contact on the other of the pair of posts to provide an electrical circuit between the posts until a predetermined threshold temperature is reached. The posts are potted into the nonconductive mounting structure by means of a thermal curable resin.
To calibrate the Gelzer switch, uncured resin is deposited in a cavity in the mounting structure and the thermal responsive switch is heated in an oven to a predetermined temperature. The post bearing the stationary contact is shifted by the bimetal element as the element is heated and deflects. After the post is appropriately positioned by the bimetal element, the heat curable resin cures, fixing the pair of posts in position in the nonconductive mounting structure. While providing a self-calibrating switch, the Gelzer calibration technique is applicable only to a switch structure in which the electrically conductive posts are held within the nonconductive mounting structure only by the thermal curable resin, and would be otherwise free to move with respect to the mounting structure. Accordingly, the fit of the posts in the nonconductive mounting structure must not be so tight as to bind the posts during the calibration operation.
It is seen, therefore, that a need exists for an electrical switching device which is simple in construction and which may be calibrated accurately without the need for manual adjustment of switch parts by a technician.